Such a process is known from Chinese patent application CN-A 1528735. This document describes a process in which carbon dioxide is reacted with an alkylene oxide to yield alkylene carbonate, e.g. propylene carbonate or ethylene carbonate. The alkylene carbonate is subjected to transesterification using an alkanol, e.g. methanol, in a reactive distillation column. In this reactive distillation column the alkylene carbonate is fed at the upper part in the liquid phase and the alkanol is fed into the column at a lower part, such that the alkanol flows upward and reacts countercurrently with the alkylene carbonate to obtain dialkyl carbonate with unreacted alkanol as the top effluent and the alkanediol with any entrained alkanol as the bottom effluent. The entrained alkanol is recycled to the reactive distillation column via a reboiler. These products from the top stream are separated in an extractive distillation unit. The extractant used in the process of CN-A 1528735 is the alkylene carbonate.
The document indicates that the energy balance in the known process is not ideal. It suggests a number of improvements. It suggests the generation of low-pressure steam from the exothermic formation of the alkylene carbonate which steam is to be used elsewhere in the process. Another suggestion is to compress part of the top stream of the extractive distillation to provide heat for the reboiler of the reactive distillation column. Although these suggestions may benefit the heat balance of the process, there is room for improvement.